A liquid-type material such as a resin suspension or cement sludge comprises volatile ingredients and non-volatile ingredients. An electrodeposition paint, for example, contains pigments and water-soluble resins as non-volatile ingredients in a volatile solvent solution which includes water and an organic solvent.
To maintain quality in working with such kinds of liquid-type material, it is important to control the concentration of its non-volatile ingredients. For example, when applying an electrodeposition paint onto such substrates as automobile body parts or building materials, the substrates are continuously immersed in an electrodeposition paint bath. During the application procedure, concentration of non-volatile ingredients in the electrodeposition paint falls gradually. Therefore, the concentration is periodically measured, and a specific quantity of the electrodeposition paint is accordingly supplied into the electrodeposition paint bath.
The concentration of non-volatile ingredients in the electrodeposition paint is usually measured according to Japanese industrial standard (JIS) K5407. This is a method comprising the steps of measuring precisely unit volume weight of the paint before and after heating, and of determining the proportion of the weight of the heated paint to that of the paint before heating. However, it takes such a long time, i.e., approximately three hours, to obtain a result by this method that it is impracticable to ascertain the real-time concentration. Consequently, when the result is obtained, the concentration of non-volatile ingredients in the electrodeposition paint bath usually has become different from that at the time of measuring, impairing accurate control of the concentration of non-volatile ingredients.
Accordingly, a variety of methods and apparatus for rapidly measuring concentration of non-volatile ingredients have been furnished. Japanese Laid-Open Patent Application No. 263297/1989, and Bander Bieche Rohre 3-1990, p. 41&ff., disclose a method for calculating concentration of non-volatile ingredients from the velocity of ultrasound passed through an electrodeposition paint. Japanese Laid-Open Patent Application Nos. 150767/1990 and 5565/1992 further disclose a method for calculating concentration of non-volatile ingredients from a damping factor of ultrasound passed through an electrodeposition paint.
However, accurate calculation of the concentration of non-volatile ingredients is impeded in the foregoing methods as well. This is because the ultrasound is scattered by air bubbles in the electrodeposition paint, generating noise. In other words, air bubbles in an electrodeposition paint interfere with the accurate measurement of both the ultrasound velocity and the damping factor.
In an electrodeposition paint, the ultrasound damping factor is linearly proportional to the concentration of non-volatile ingredients if pH, temperature and solvent concentration of the electrodeposition paint are stable, such that it is possible to ascertain accurately the concentration of non-volatile ingredients in the paint immediately from the damping factor, if the relationship between damping factor and concentration of non-volatile ingredients is previously determined. However, in an actual measuring procedure, the pH and other factors may not be stabilized, such that the damping factor will not always be linearly proportional to the concentration of non-volatile ingredients; it thus becomes unfeasible to determine accurately the concentration of non-volatile ingredients only from the damping factor.